x86: dm: Clean up cpu drivers

This commit does the following to clean up x86 cpu dm drivers:
- Move cpu_x86 driver codes from arch/x86/cpu/cpu.c to a dedicated
  file arch/x86/cpu/cpu_x86.c
- Rename x86_cpu_get_desc() to cpu_x86_get_desc() to keep consistent
  naming with other dm drivers
- Add a new cpu_x86_bind() in the cpu_x86 driver which does exactly
  the same as the one in the intel baytrail cpu driver
- Update intel baytrail cpu driver to use cpu_x86_get_desc() and
  cpu_x86_bind()

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/cpu/Makefile b/arch/x86/cpu/Makefile
index 7ff05e6..48197fb 100644
--- a/arch/x86/cpu/Makefile
+++ b/arch/x86/cpu/Makefile
@@ -10,7 +10,7 @@
 
 extra-y	= start.o
 obj-$(CONFIG_X86_RESET_VECTOR) += resetvec.o start16.o
-obj-y	+= interrupts.o cpu.o call64.o
+obj-y	+= interrupts.o cpu.o cpu_x86.o call64.o
 
 obj-$(CONFIG_INTEL_BAYTRAIL) += baytrail/
 obj-$(CONFIG_SYS_COREBOOT) += coreboot/